I understand that there is one provided as a sample in the Windows SDK.
The BCX forum has source code for the Easy IDE (written in BCX) which you can run through the BCX translator for the C code.
If you end up pulling from different sources to build a dialog editor might I recommend my property grid for the property editor? The source code for POINST Wizard demonstrates how I implemented it to change the properties of the wizard project.
Dmac